The present invention relates generally to hydraulic circuit systems and, more specifically, to hydraulic systems having a priority and a second fluid circuit supplied primarily by a variable displacement pump and a fixed displacement pump, respectively.
The present invention is applicable to agricultural and industrial vehicles wherein one hydraulic circuit is provided for use in steering the vehicle and a second hydraulic circuit is provided for use in operating a hydraulic implement or accessory. In many working operations, the steering circuit of the tractor is not used or is operated at less than maximum capacity. It is well known in such applications to mount two fixed displacement pumps in tandem in a hydraulic circuit and control the output of these pumps through the use of various valving arrangements. The pumps supplying fluid to the steering circuit have generally been of the constant delivery or fixed displacement type. However, the use of fixed displacement pumps becomes uneconomical in an application that requires high pressure and negligible flow for any sustained period of time since the fixed displacement pump will continue to deliver its full fluid flow even when only minimal flow is needed. In order to save horsepower and be more energy efficient, alternative fluid supply means have been sought which only apply pressure and fluid flow as needed at particular points in time.
One such alternative means to using a fixed displacement pump in a hydraulic system is to use a variable displacement pump whose stroke can be adjusted to fill the need of either high volume or high pressure, as required. Variable displacement pumps have become more acceptable in mobile hydraulics today for a variety of reasons. More important among these reasons are more competitive unit costs and energy efficiency with respect to fixed displacement pumps. Since engines on vehicular equipment traditionally have a speed spectrum, the variable pump may be sized and controlled to provide proper flow at both extremes of the spectrum. Undersizing or oversizing efficiency losses, inherent in fixed displacement pumps used in equipment having a speed spectrum, may thus be eliminated by use of variable displacement pumps.
The size chosen for a variable displacement pump is usually a compromise between cost and performance. As such, pumps are often too small to quickly meet large demands for flow. A high flow circuit, such as a steering circuit of a vehicle, has such demands. In an open center steering circuit, at low engine speeds, output flow from a normally chosen, small, variable displacement pump is insufficient to provide enough fluid flow to achieve desired lock-to-lock time. An operator feels the steering as "too hard".
Operator effort is lessened by increased flow from a second, fixed displacement pump. While not in use, the output of the fixed displacement pump is passed via an unloading valve to tank. Such a design is inefficient since it makes minimal use of the second pump. Other open center implement systems experience similar inadequacies when large flows are needed for quick implement response.
Closed center steering or implement circuits have similar flow demands. For example, a closed center steering system normally includes an accumulator. For desired operation, the system requires that the accumulator be quickly filled. Again, a variable displacement pump may be unable to meet the fast filling requirement of the system accumulator, so a further pump is required.
To dispense with a second pump, the variable displacement pump may be oversized to meet occasional large flow demands or a small pump can be modified to deliver substantial flow at 1 engine speeds. The displacement control of a small pump can be set with a stop to deliver flow at a rate approximating the behavior of a fixed displacement pump, but such a modified pump still wastes energy when the priority circuit is idle.
So, it would be desirable to have a more efficient and versatile system that uses a fixed displacement pump and a variable displacement pump where the output of the fixed displacement pump in proportion to the flow demanded by a priority (e.g. steering or implement) circuit. In such a desired system, the fixed displacement pump would normally be available for other uses, but would be diverted to the priority circuit upon a sensed demand for flow.